1. Field of the Invention
The present invention relates to a spread spectrum communication apparatus to be used in a communication system which carries out radio communication with a base station by a spread spectrum communication method and to which incoming information is sent from the base station at a known timing during waiting, and to a of controlling method the spread spectrum communication apparatus.
2. Description of the Related Art
In a mobile communication terminal to be used in a mobile communication system, discontinuous receiving for battery saving has been widely carried out.
The discontinuous receiving may be in an operating state in which incoming information can be normally received at a timing when there is the possibility that incoming information intended for that apparatus will come. The periods except for the operating periods therefor are sleeping periods. In the sleeping period, operation except for that of one part of the circuits is stopped. It can be considered that monitoring of a timing of rising from the sleeping period is carried out by clocking based on a clock signal.
The mobile communication terminal generally uses a high-speed and high-accuracy clock signal for a precise synchronization for communication. When such a clock signal is used, the rising timing can be high accurately monitored. However, in a high-speed oscillator which oscillates the high-speed and high-accuracy clock signal, the electric power consumption is high.
Therefore, a low-speed oscillator which is low-speed and has low-accuracy, as compared with the high-speed oscillator, is provided. Monitoring of the rising timing is carried out on the basis of a low-speed and low-accuracy clock signal to be oscillated by the low-speed oscillator. Further, it is desirable to attempt a further decrease in electric power consumption by stopping the operation of the high-speed oscillator during the sleeping period.
However, when a low-speed and low-accuracy clock signal is used for clocking of the sleeping period, offset will occur in the determination of the rising timing. Further, when the rising timing is offset, there is the fear that the incoming information will fail to be received.